The invention relates to a device for effecting reverse osmosis, provided with a composite dynamic membrane, which comprises a porous supporting device, an intermediate layer of a material that is hardly or not deformable, which layer may also be a multi-layer, and a layer of membrane-forming material. The invention also relates to a method for preparing such a membrane.
The application of such an intermediate layer of a material that is hardly or not deformable has important advantages. If the membrane-forming material should be deposited on the porous supporting layer directly then this supporting layer must have very fine pores, as otherwise the membrane-forming material passes through the pores or deposits in the pores, so that no or unsufficient membrane-forming on the surface of the supporting layer takes place. A supporting layer with very fine pores has as drawbacks, however, that the material of the supporting layer is expensive and that the colloidal particles cause a clogging of this layer that can hardly or not be removed. These drawbacks may be removed by applying between the supporting layer and the membrane-forming material a layer that has smaller pores than the supporting layer, so that after all a supporting layer with relatively large pores can be used. The porous supporting device mostly employed is a set of tubes of porous ceramic material.
This intermediate layer, which forms the substratum for the membrane-forming material, must meet some conditions;
Firstly, the material of the intermediate layer must be hardly or not deformable; secondly, the structure must be such that a flux through the composite membrane that is as great as possible, is attained and thirdly, when particles are used as a membrane-forming material, as little leakage as possible must occur between the membrane particles and the intermediate layer.
In the dynamically formed membranes known so far in this field of art that are provided with one intermediate layer, all these three conditions are not met simultaneously. It is also known in the art that an intermediate layer has been applied that is composed of three layers, having a ratio of the respective particle diameters of 1:2:3, the layer with the largest particle diameter lying on the supporting layer. As a result, the pores are not reduced in size sufficiently, however, so as to obtain an adequate membrane in all cases.
The best result has been obtained so far by applying instead of an intermediate layer, a membrane filter, e.g. comprising paper, fabric or microporous plastic, but nevertheless, the properties of such a membrane filter are also far from optimum, whereas, moreover, a membrane formed in such a way cannot be rinsed any longer.